Process for the preparation of rare earth oxyhalide phosphor

ABSTRACT

Rare earth oxyhalide phosphors activated with thulium ion and further incorporating scandium ion are described exhibiting improved optical resolution for the light image produced therefrom in X-ray image converter devices by reason of the particle size and shape of the phosphor crystals. A multilayer X-ray screen construction utilizing said phosphor material is also described exhibiting improved image sharpness attributable to said phosphor improvement. A preparation method is disclosed wherein the selected oxyhalide is recrystallized in a particular manner to produce phosphor crystals having a more polyhedral shape.

This application is a continuation-in-part of application Ser. No.483,335, filed April 8, 1983, now abandoned.

RELATED APPLICATIONS

Patent application Ser. No. 691,323, filed concurrently herewith, isanother continuation-in-part application of said aforementioned parentapplication as is also patent application Ser. No. 691,324, also filedconcurrently herewith.

BACKGROUND OF THE INVENTION

The present invention pertains to preparation of particular rare earthoxyhalide phosphor materials activated with thulium exhibiting superioroptical resolution for the light image produced therefrom when used inX-ray image converter devices. Conventional phosphors of this type stillproduce considerable loss in image resolution when the particle size ofthe phosphor crystals exceeds approximately 10 microns diameter and suchoverly coarse phosphor crystals are generally attributable to the methodof phosphor preparation. It would be desirable, therefore, to provide animproved method to prepare these phosphor materials which produces stillbetter shaped phosphor crystals of smaller particle size for resultingimprovement in this product application.

Recrystallization is an already well recognized method of controllingthe particle size of inorganic phosphor crystals. For rare earthoxyhalide phosphors, there is disclosed in U.S. Pat. No. 3,591,516 whichis assigned to the assignee of the present invention, a method toproduce well formed crystals of these oxyhalide materials byrecrystallization in molten alkali metal halide fluxes but the particlesize of the phosphor crystals is reported to exceed 10 microns diameter.Undesirable particle size increase for these thulium activated phosphorshas now been discovered with decreasing activator level. Moreparticularly, in my recently issued U.S. Pat. No. 4,478,933 there isdisclosed a class of said rare-earth oxyhalide phosphors activated withthulium ion alone at a concentration in the range from about 0.0005 to0.001 mole per mole of the phosphor which exhibits reduced crossovereffects. Unfortunately, the average particle size range in said phosphormaterial has been found to exceed 10 microns diameter at the lower endof said activator range and with added difficulties being experienced inmaintaining the particle size range at lower values by reducing therecrystallization temperatures. What has been discovered in lowering therecrystallization temperatures when preparing said phosphor material inmolten alkali metal halide fluxes is that a greater variation occurs inthe recrystallized phosphor particle size due to a variety of suchfactors as viscosity variation of the molten flux, localized temperaturefluctuations, and still other undersirable effects.

In still another recently issued U.S. Pat. No. 4,315,979, also assignedto the present assignee, there is disclosed a recrystallization methodfor these phosphor materials using an alkali metal halide flux mixturewhose halides correspond to the selected oxyhalide phosphor and carryingout said recrystallization at temperatures above the eutectic meltingtemperature of the flux mixture. In an alternate method of said phosphorpreparation, the alkali metal carbonates of the selected alkali metalhalides are added as starting materials before the initial heating stepso that said alkali metal halides are formed in situ during said initialheating step. The phosphor crystals obtained in this manner are said toconsist essentially of recrystallized phosphor crystals having anaverage median particle size range not below about 2 microns and notgreater than about 16 microns for less light scattering and lightabsorption.

It would be desirable, therefore, to still further reduce the particlesize and particle size distribution of this phosphor material during thephosphor preparation for improved optical resolution of X-ray imageconverters using this phosphor. It would also be desirable to do so inthe manner not requiring elaborate modification of the phosphorpreparation method now commonly used for superior X-ray image converterperformance in other respects. It would be still further desirable toalter the shape of the individual phosphor particles during preparationas additional means to provide said improved performance.

SUMMARY OF THE INVENTION

A superior X-ray image converter is herein provided comprising wellformed recrystallized phosphor crystals having the structural formula;

    LnOX:Tm.sup.3+, Z

wherein

Ln is one or more of La and Gd,

X is one or more of Br and Cl,

Tm is present as an activator at a concentration in the range from about0.0005 to 0.01 mole per mole of the selected oxyhalide,

Z is scandium ion which substitutes for Ln ion in an amount from about0.001 up to approximately 0.2 mole per mole of phosphor,

and with said recrystallized phosphor crystals having an average medianparticle size in the range extending from approximately 3.0 micronsdiameter up to approximately 9.0 microns diameter along with a morenarrow particle size distribution. Preparation of said improved phosphormaterial can be obtained by heating a mixture of the rare earth oxidesof the selected oxyhalide which further includes said thulium andscandium ion along with an ammonium halide whose halide is that of theselected oxyhalide for a time and temperature sufficient to form theselected oxyhalide phosphor and thereafter reheating the phosphormaterial with an alkali halide flux whose halide is again that of theselected oxyhalide to a temperature not exceeding approximately 900° C.for a time sufficient to recrystallize said selected oxyhalide phosphor.An alternate method of phosphor preparation mixes alkali metalcarbonates whose halides are that of the selected oxyhalide with thestarting materials before the initial heating step so that said alkalimetal carbonates react with said ammonium halide to form an alkalihalide flux mixture during the initial heating step.

In the present phosphor preparation, such utilization of scandium ioncontrols the particle size to a dramatic degree. Lower scandium ionlevels than hereinbefore specified have not demonstrated this effectwhereas a greater scandium level than specified has an adverse effect ofreducing phosphor brightness. Since the scandium ion used in saidphosphor preparation replaces part of the rare earth ion in the phosphormatrix there is achieved in said manner both a novel phosphor materialtogether with a novel method for its preparation. In the former respect,it can be noted that the scandium level in the improved phosphor canconsiderably exceed the thulium activator level. In said latter respect,the scandium ion forms a relatively non-volatile halide in the moltenalkali metal halide flux used in the phosphor recrystallization step. Asabove generally pointed out, said improved phosphor preparation requiresheating a particulate oxide mixture selected from lanthanum andgadolinium oxides, including mixtures thereof, which further containsthe thulium activator ion and ammonium halide whose halide ion is thatof the selected oxyhalide along with sufficient scandium ion to producea scandium level in said phosphor from about 0.001 to 0.2 mole per moleof the phosphor for a time and temperature sufficient to form theselected oxyhalide phosphor, and thereafter reheating said phosphormixture with an alkali halide flux mixture whose halide ions are alsothat of the selected oxyhalide to a temperature not exceedingapproximately 900° C. for a time sufficient to recrystallize saidselected oxyhalide phosphor.

Preferred rare earth oxyhalide phosphor materials of the presentinvention having the above specified replacement scandium ion in thephosphor matrix at the specified concentration levels comprise lanthanumand gadolinium oxybromide activated with thulium ion again in thespecified concentration range. including mixtures thereof. In apreferred embodiment for preparation of said thulium activated phosphormaterial, 378 grams of La₂ O₃ :0.001Tm is dry blended with 0.10 gram SC₂O₃, 14.0 grams Li₂ CO₃, 10.0 grams K₂ CO₃, and 334 grams NH₄ Br and theblended mixture next fired in a covered vessel at approximately 400° C.for about 2 and 1/2 hours to form the phosphor product. Said phosphorproduct is next blended and fired in a covered vessel for an additional2 and 1/2 hours at appproximately 800° C. The recrystallized phosphor isfinally washed free of water soluble salts after filtration and dryingto produce a final product with the structural formula:LaOBr:0.OO1Tm,O.OO1Sc. The particle size of said phosphor crystals, asmeasured by Coulter Counter instrument, was found to be an averagemedian particle size (APD) of approximately 3.7 microns diameter.

Surprisingly, further observation of said phosphor crystals found theeffect of the replacement metal ion to be retardation of crystal growthin the fastest crystal growth direction and which resulted in crystalshaving a still greater thickness than reported in the above mentionedU.S. Pat. No. 4,315,979. Such a thickness increase in the now generallysmaller size and more polyhedral shaped phosphor crystals reduces thevarious optical resolution difficulties which are explained in saidaforementioned patent as attributable to the flat plate like phosphorcrystals therein obtained. Thus, not only is there experienced greateroptical resolution attributable to the smaller particle size of thepresently improved phosphor crystals but less light scattering as well.Said latter improvement reduces the light crossover problem still beingencountered in X-ray image converter devices using the conventionallyshaped phosphor crystals.

In the preferred X-ray screen constructions, the modified phosphors ofthe present invention are employed as a pair of phosphor layers whichare positioned on each side of a double emulsion photographic film toform a sandwich configuration. The preferred multi layer X-ray screenfurther incorporates a UV absorption dye in the otherwise transparentlayer of the photographic film member to cooperate with the presentphosphor materials in reducing the amount of emitted radiation which cancrossover to the more remote emulsion layer. Crossover widens therecorded image and produces blurring due to lack of alignment betweenthe image formed on the nearest emulsion layer and the crossover imageproduced the more remote emulsion layer. Understandably, the presentmodified phosphors produce a light image having better opticalresolution when recorded upon the associated photograpic film due toimproved size and form of these more polyhedral shaped phosphorcrystals.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing there is shown a cross sectional view of amulti layer X-ray screen utilizing the present modified phosphormaterials along with a dye system to absorb ultraviolet light whichotherwise crosses over to the remote emulsion layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Said accompanying drawing depicts a double emulsion photographic film 9which is sandwiched between two X-ray intensifying screens. The screensare constructed in a conventional manner with a flexible backing 5 whichsupports a pair of reflecting layers 6, a pair of phosphor layers 7 towhich this invention relates and a pair of transparent layers 8. It isto be pointed out that the particular construction of said X-ray screenmember is merely exemplary and thereby is not intended to be alimitation on the invention which resides in the modified phosphormaterial improving the overall device performance. Accordingly, there isprovided in Table 1 below representative examples evidencing control ofphosphor particle size by the level of scandium replacement ion ionincorporated into the phosphor matrix according to the presentinvention. The particular phosphor being modified (LaOBr:O.OO1Tm) wasprepared in the same manner previously described to include the scandiumconcentrations listed in said Table along with the respective ionicradius value for said replacement metal ion.

                  TABLE 1                                                         ______________________________________                                                 Metal Ion                                                                     (Mole/Mole  Coulter APD                                                                              Ionic Radius                                  Example  Phosphor)   (Microns)  (Angstroms)                                   ______________________________________                                        1        None        9.6        --                                            2        .001 Scandium                                                                             3.7        .73                                           3        .002 Scandium                                                                             2.8        .73                                           ______________________________________                                    

It can be noted from said reported particle size values in examples 2and 3 that increasing the level of replacement scandium ion in thephosphor matrix produces greater reduction in the phosphor particlesize. That a greater degree of uniformity in particle size can beachieved with the present method of preparation is evidenced with testresults wherein the particle size was maintained between 4 micronsdiameter and 8 microns diameter. From said results it seem furtherevident that such relatively modest modification in the phosphor matrixproduces a pronounced effect upon both particle size and shape in therecrystallized phosphor product.

It will be apparent from the foregoing description that novel phosphormaterials and X-ray image converter devices utilizing said phosphormaterials have been disclosed exhibiting significant performanceimprovement. It should be appreciated from said foregoing description,however, that comparable phosphors can be prepared having compositionsother than above specifically disclosed in the same manner.Additionally, other recrystallization flux mixtures disclosed in theaforementioned U.S. Pat. No. 4,315,979 are contemplated to produce stillfurther performance improvement in the particle size and shape of theserecrystallized phosphor crystals. It is intended to limit the presentinvention, therefore, only by the scope of the following claims.

What I claim is new and desire to secure by Letters and Patents of theUnited States is:
 1. A process for the preparation of lanthanum andgadolinium oxyhalide phosphor crystals selected from the groupconsisting of oxychlorides and oxybromides activated with thulium ionconsisting essentially of recrystallized phosphor crystals having anaverage median particle size in the range extending from approximately3.0 microns diameter up to approximately 9.0 microns diameter along witha more narrow particle size distribution and polyhedral shape comprisingthe following steps:(a) heating a mixture of particles of oxidesselected from lanthanum and gadolinium oxides, including mixturesthereof, which further contains thulium ion in activator amounts fromabout 0.0005 to 0.01 mole per mole of the phosphor and an ammoniumhalide whose halide ion is that of the selected oxyhalide, along withsufficient scandium ion to produce a scandium level in said phosphorfrom about 0.001 to 0.2 mole per mole of the phosphor for a time andtemperature sufficient to form the selected oxyhalide phosphor, and (b)reheating said phosphor with an alkali metal halide flux mixture whosehalide ions are that of the selected oxyhalide phosphor to a temperaturenot exceeding approximately 900° C. for a time sufficient torecrystallize said selected oxyhalide phosphor.
 2. A process as in claim1 wherein at least one alkali metal carbonate is included in the mixtureemployed in step (a) so that said alkali metal carbonate reacts withsaid ammonium halide to form one of the selected alkali metal halides ofthe flux mixture in the initial heating step.
 3. A process as in claim 1wherein the scandium ion forms a relatively non-volatile halide in thealkali metal halide flux mixture.